1. Field of the Invention
The present invention relates generally to an apparatus for reproducing a disc record on which a digital signal is recorded in which the rotational velocity of the disc record is controlled, and is directed more particularly to an apparatus for reproducing a disc record in which the linear velocity of the disc record is maintained at a constant predetermined value.
2. Description of the Prior Art
At present, a digital-audio disc record has been developed in which a PCM (pulse code modulation) audio signal is recorded on a disc record similar to a video disc and then reproduced. As the reproducing system thereof, a signal detecting system of a mechanical type, optical type, electro-static capacity type or the like similar to that for the video disc record is used. When a PCM audio signal is recorded on a disc record, order to increase the recording density, the linear velocity of the disc record is selected to be constant, rather than selecting the angular velocity of the disc record to be constant. When the disc record on which a signal is recorded with a constant linear velocity is reproduced, it is of course necessary that the disc record be reproduced at a constant linear velocity. To this end, a rotational control method for the disc record is known in which the position of a pick-up device is detected by a potentiometer and, since the necessary number of rotations is a reciprocal of the position of the pick-up device, the detected output therefrom is supplied to a divider to obtain control information or data. However, the above method is not so good because the position detector and divider for the control are expensive and complicated in construction.
Further, a method for obtaining the control information or data based upon the reproduced signal from the disc record has been proposed in which the fact that the reproducing system of the optical disc record has a frequency characteristic similar to that of a low pass filter and the rising-up or falling-down time of the reproduced signal is in inverse proportion to the linear velocity of the disc record. The reproduced signal is differentiated and the control information is obtained from the peak value of the differentiated output (as disclosed, for example, in U.S. patent application Ser. No. 204,552).
Further, since the cut-off space frequency of the reproducing system of the optical disc record is determined by the open area ratio of the objective lens used for reading-out the disc record and the wave length of a laser beam used therewith, a troublesome or complex manner of adjustment must be made for each player or change of the pick-up device.
Further, when disc records having a different linear velocity upon recording are reproduced, adjustment is also necessary.